After decades of American dominance in space exploration, the next-to-last shuttle flight brings country to the threshold of a period that experts are calling "The Gap," -- the first significant stretch of time in decades during which the U.S. will be unable, on its own, to put astronauts into space.

"I don't like it at all," said Rep. C.A. Dutch Ruppersberger, a Maryland Democrat who has led oversight of the space program. "The previous administrations have not made space a priority. It's expensive. Now we're in this situation."

If the fears of some in Congress come true, a period of unprecedented drift for the space program could follow the final Shuttle launch, now scheduled for July. With no American vehicle capable of carrying astronauts into space, the U.S. will be forced to pay the Russians a steadily escalating price -- eventually hitting $62.7 million per seat -- to carry Americans and international partners to the International Space Station through 2016.

Meanwhile, NASA Administrator Charles Bolden told ABC News that the U.S. will be relying on a relatively young collection of private companies to build the rockets that will restart American-led missions to the space station, which he estimates will begin launching by 2015.

"Everybody knew it was coming," Bolden said of The Gap. "The primary hurdle it creates is that people will become comfortable with it. We tend to be short-sighted and our memory is short."

NASA officials are quick to note that under the Bush administration's space initiative, known as Constellation, The Gap would have lasted eight years. A six-year gap, if all goes as planned, would pass more quickly than the eight-year gap between the end of the Apollo program and the launch of the first space shuttle in 1981.

The public posture of NASA officials has been to focus on a modernized program that relies far more on private companies to handle the increasingly routine work of hoisting satellites and servicing the space station, while dedicating U.S. government resources to planning the more complex task of taking astronauts deeper into space. Bolden says NASA will be developing a separate, heavy-lift rocket to explore deep space and eventually, maybe, take astronauts to an asteroid, the moon, and Mars.

But privately, political leaders are bemoaning what could be a deeply unsettling period during which the U.S. will have no way to put humans into space -- and efforts to reach more distant destinations appear hazy and uncertain. Bolden's critics told ABC News they see significant hurdles for NASA after the shuttle retires. They are worried that without a clear destination or proven spacecraft to get there, it could be a long, long time before a manned U.S. rocket heads for the heavens.

"What used to be a gap is now a cliff," said Michael D. Griffin, who served as NASA administrator from 2005 to 2009 under President Bush. "What really is happening here is the destruction of an American institution that has been preeminent in the world for the past 40 years. I believe it's tragic."

Obama Scraps Moon Plans

Griffin's chief complaint is that the Obama administration has scrapped the plans sketched out during the Bush years for the development of new spacecraft to take humans back to the moon. And he and others find excruciating the idea that the U.S. could be completely dependent for the next several years on Russian Soyuz rockets to carry astronauts to the International Space Station.

The U.S. has signed a series of agreements with the Russians. The most recent, announced last month, included a sizeable price hike. And because the U.S. had earlier pledged to carry space station partners from Japan, Canada, and Europe, American taxpayers will pay the Russians for those seats as well.

"They're exercising their leverage," Griffin told ABC News. "We knew the minute the shuttle stopped flying the price on the Russian craft was going to go up. We told them capitalism was a better path. Well they have embraced that. The Russians are now in the catbird seat and they're going to charge for it."

Bolden disagreed, calling the Russians "very reliable partners." At the same time, he added, "I don't want anyone to get comfortable with that partnership and rely on them."

The NASA administrator expressed confidence in the work of SpaceX, the California company started by internet billionaire Elon Musk, one of two private firms working on plans to carry supplies, and eventually astronauts, back and forth to the space station. The company has had a successful test flight, during which it put a capsule into orbit and then returned it safely to earth.

The company still has two more tests to complete before NASA allows it to carry cargo to the station as part of a multi-billion dollar contract. But Bolden hinted he may be preparing to permit SpaceX to speed up its work by conducting the two tests (one includes successfully docking with the station) during a single flight.

"NASA will no longer procure vehicles and operate them for low earth orbit activities," Bolden said. "We are going to completely rely on [commercial companies] for that work."

In Congress, however, there continues to be skepticism about that approach.

"Until commercial companies demonstrate the ability to carry cargo to space – as taxpayers have already paid them to do -- why would we trust them to carry human beings there?" Sen. Richard Shelby, an Alabama Republican, told ABC News in an email.

Sen. Bill Nelson, a Florida Democrat, said he believes private commercial space companies are making progress.

"We have begun to see test flights of new vehicles and new spacecraft," he said. "But, spaceflight is a risky business and these companies still have a long way to go before they are ready to put U.S. astronauts on board."

Scott Pace, a former NASA official who now oversees the Space Policy Institute at George Washington University, said the push towards relying more on commercial companies makes sense. But he is concerned that Bolden is making NASA over-reliant on commercial businesses.

"I think SpaceX is a great company," Pace said. "My problem is, I have a criticism of a government policy that rides completely on the success of that company. The history of space entrepreneurial firms is that the vast, vast majority fail. It doesn't make a lot of logical sense to me."

Bolden said that while he has confidence in the agency's new commercial partners, he is approaching the partnership with a certain degree of caution. NASA will be an active partner as plans develop to put astronauts back into space, he said, not a bystander.

"I am very optimistic," he said. "Space exploration has a bright future. We're just at the beginning."